Dispersion shifted optical fiber having triple clad

Abstract

The present invention is related to a dispersion-shifted optical fiber for use in a wavelength division multiplexing system, which comprises a core, an inner clad, and a silica clad, wherein the optical fiber has a relatively low dispersion at a wavelength of 1550 nm, an effective area of 60 mum2 and small bending loss. The core consists of a first and a second core, wherein the core has a refractive index distribution. The inner clad consists of a first inner clad having a refractive index smaller than that of the second core and a second inner clad having a refractive index smaller than that of the second inner clad. A silica clad surrounds the inner clad and has a refractive index smaller than that of the second inner clad.

What is claimed is:  
     
       1. A dispersion shifted optical fiber, comprising: 
       a core having a refractive index distribution (n core );  
       an inner clad surrounding the core and having a refractive index distribution (n clad ); and,  
       a silica clad surrounding the inner clad and having a refractive index (n silica );  
       wherein the core comprises:  
       a first core having a refractive index (n 1 ) that is uniform over its radius (a 1 ); and  
       a second core surrounding the first core and having a refractive index (n 2 ) that is gradually decreased over its radius (a 2 ); and,  
       wherein the inner clad comprises:  
       a first inner clad surrounding the second core and having a refractive index (n 3 ) that is uniform over its radius (a 3 ); and  
       a second inner clad surrounding the first inner clad and having a refractive index (n 4 ) that is uniform over its radius (a 4 ); and,  
       wherein the refractive indices of the first and second cores, the first and second clads and the silica clad satisfy a relationship of n 1 >n 2 >n 3 >n 4 >n silica ;  
       wherein the optical fiber has dispersion of the 1.5˜3.5 ps/nm-km at a wavelength of 1550 nm, an effective area of 60˜70 μm2 and a 32 mmψ bending loss of 0.02 dB or less;  
       wherein a radius of the first core (a 1 ), a radius of the second core (a 2 ), a radius of the first inner clad (a 3 ) and a radius of the second inner clad (a 4 ) are 1.0±0.15 μm, 2.3±0.15 μm, 6.5±1.5 μm and 9.0±1.0 μm, respectively; and refractive index differences del 1 (%), del 2 (%) and del 3 (%) are 0.73±0.03%, 0.11±0.03% and 0.07±0.03%, respectively, wherein del 1 (%) is calculated from the formula (1) below; del 2 (%) is calculated from the formula (2) below; and del 3 (%) is calculated from the formula (3) below;  
       
         
             del   1 (%)=[( n   1 − n   silica )/ n   silica ]×100;  (1)  
         
       
       
         
             del   2 (%)=[( n   2 − n   silica )/ n   silica ]×100;  (2)  
         
       
         del   3 (%)=[ n   3 − n   silica )/ n   silica ]×100  (3). 
     
     
       2. The optical fiber as claimed in  claim 1 , wherein the refractive index (n 2 ) of the second core is linearly decreased over its radius (a 2 ). 
     
     
       3. A dispersion shifted optical fiber, comprising: 
       a core having a refractive index distribution (n core ):  
       an inner clad surrounding the core and having a refractive index distribution (n clad ); and,  
       a silica clad surrounding the inner clad and having a refractive index (n silica );  
       wherein the core comprises:  
       a first core having a predetermined refractive index (n 1 ) that is gradually increased from an initial refractive index (n 0 ) over its radius (a 1 ); and  
       a second core surrounding the first core and having a refractive index (n 2 ) that is gradually decreased over its radius (a 2 ); and,  
       wherein the inner clad comprises:  
       a first inner clad surrounding the second core and having a refractive index (n 3 ) that is uniform over its radius (a 3 ); and  
       a second inner clad surrounding the first inner clad and having a refractive index (n 4 ) that is uniform over its radius (a 4 ); and  
       wherein a maximum refractive index of the second core is larger than a minimum refractive index of the first core and the refractive indices of the second core, the first and second inner clads and the silica clad have a relationship of n 2 >n 3 >n 4 >n silica ;  
       wherein the optical fiber has the dispersion of 1.5˜3.5 ps/nm-km at a wavelength of 1550 nm, an effective area of 60˜70 μm2 and a 32 mmψ bending loss of 0.02 dB or less;  
       wherein a radius of the first core (a 1 ), a radius of the second core (a 2 ), a radius of the first inner clad (a 3 ) and a radius of the second inner clad (a 4 ) are 0.85±0.15 μm, 2.7±0.15 μm, 7.0±1.5 μm and 9.0±1.0 μm, respectively; and refractive index differences del 0 (%), del 1 (%), del 2 (%) and del 3 (%) are 0.70±0.03%, 0.75±0.03%, 0.10±0.02% and 0.06±0.03%, respectively, wherein  
       del 0 (%) is calculated from the formula (1) below; del 1 (%) is calculated from the formula (2) below; del 2 (%) is calculated from the formula (3) below; and, del 3 (%) is calculated from the formula (4) below;  
       
         
             del   0 (%)=[( n   0 −n silica )/ n   silica ]×100;  (1)  
         
       
       
         
             del   1 (%)=[( n   1 −n silica )/ n   silica ]×100;  (2)  
         
       
       
         
             del   2 (%)=[( n   2 −n silica )/ n   silica ]×100;  (3)  
         
       
       
         
             del   3 (%)=[( n   3 −n silica )/ n   silica ]×100  (4).  
         
       
     
     
       4. The optical fiber as claimed in  claim 3 , wherein the refractive index (n 2 ) of the second core is linearly decreased over its radius (a 2 ). 
     
     
       5. The optical fiber as claimed in  claim 1 , wherein a ratio of a radius of the first inner clad (a 3 ) to a radius of the second inner clad (a 4 ) ranges from 0.5 to 0.8. 
     
     
       6. The optical fiber as claimed in  claim 1 , wherein the index difference del 2 (%) minus the index difference del 3 (%) ranges from 0.04% to 0.06%. 
     
     
       7. A dispersion shifted optical fiber, comprising: 
       a core having a refractive index distribution (n core );  
       an inner clad surrounding the core and having a refractive index distribution (n clad ); and,  
       a silica clad surrounding the inner clad and having a refractive index (n silica );  
       wherein the core comprises:  
       a first core having a refractive index (n 1 ) that is uniform over its radius (a 1 ); and  
       a second core surrounding the first core and having a refractive index (n 2 ) that is gradually decreased over its radius (a 2 ); and,  
       wherein the inner clad comprises:  
       a first inner clad surrounding the second core and having a refractive index (n 3 ) that is uniform over its radius (a 3 ); and  
       a second inner clad surrounding the first inner clad and having a refractive index (n 4 ) that is uniform over its radius (a 4 ); and,  
       wherein the refractive indices of the first and second cores, the first and second clads and the silica clad satisfy a relationship of n 1 >n 2 >n 3 >n 4 >n silica ;  
       wherein the optical fiber has disperson of the 1.0˜6.0 ps/nm-km at a wavelength of 1550 nm, a dispersion slope of 0.06 ps/nm2-km, an effective area of 50 μm2 or more and a 32 mmψ bending loss of 0.01 dB or less at 1550 nm and 1620 nm wavelengths;  
       wherein a radius of the first core (a 1 ), a radius of the second core (a 2 ), a radius of the first inner clad (a 3 ) and a radius of the second inner clad (a 4 ) are 1.0±0.15 μm, 2.5±0.15 μm, 3.5±1.5 μm and 6.0±1.0 μm, respectively; and refractive index differences del 1 (%), del 2 (%) and del 3 (%) are 0.75±0.03%, 0.15±0.02% and 0.10±0.03%, respectively, wherein  
       del 1 (%) is calculated from the formula (1) below; del 2 (%) is calculated from the formula (2) below; and del 3 (%) is calculated from the formula (3) below;  
       
         
             del   1 (%)=[( n   1 −n silica )/ n   silica ]×100;  (1)  
         
       
       
         
             del   2 (%)=[( n   2 −n silica )/ n   silica ]×100;  (2)  
         
       
       
         
             del   3 (%)=[( n   3 −n silica )/ n   silica ]×100  (3).